Fuel injection nozzles are equipped as so-called needle-stroke transducers, in order to indicate the onset of injection by means of a signal which is carried further to a testing device. The opening and closing of an electrical current circuit--that is, switching between zero voltage and the supply voltage--by a switch actuated by the nozzle needle is made use of in many cases, because the switch can be easily built into mass-produced injection nozzles without requiring extra space.
It is particularly favorable to bring the valve seat of the nozzle body and the valve cone of the nozzle needle into play as a mechanical switch, but in that case, there are problems of insulation which are not yet satisfactorily solved.